Madonna looks out from the stage and takes in the sight of central London at the height of summer. With a certain inevitability, the sky looks like it's been cast in concrete and it's tipping down. "Maybe," she suggests gamely, "if we put our minds together we can stop it raining."

The crowd cheer damply, but it's hard to avoid the conclusion that the weather is fairly low down on the list of problems with the London debut of the MDNA tour. Our economy may be tanking, our global influence ever diminishing, but when it comes to standing around in the rain at outdoor gigs, the British remain the undisputed world leaders. The real issue here is the sound, or rather lack of it. As if to demonstrate to Bruce Springsteen fans that their decision to pull the plug on him was nothing personal – they're fully equipped to ruin gigs by pop stars as well as venerable rock legends – the authorities have set it at a volume so feeble it sounds less like a live performance than something being sodcasted through the internal speaker on a mobile phone. It's a peculiar sensation, watching a woman who's clearly busting a gut up there – waving a gun around, pretending to dispatch assassins, being trussed and gagged by her backing dancers, at one point actually walking a tightrope – to a literally muted effect. She makes a little speech about the dangers of discrimination and prejudice. It's clearly hugely well-intentioned, the kind of thing that would ordinarily get everyone cheering and punching the air. Alas, thanks to the sound issue, its reception in the crowd is weirdly reminiscent of The Sermon on the Mount in Monty Python's Life of Brian: "What did she say? Blessed are the cheesemakers?"

The inaudibility is not Madonna's fault, but it's compounded by her choice of material. When she performs Vogue or Express Yourself – the latter interpolated with a burst of Lady Gaga's Born This Way that you might mistake as a respectful nod until she follows it by singing "she's not me, she's not me" while exposing her left buttock to the crowd – the sheer power of the song manages to transcend the weediness of the sound. But the show is bullishly heavy on the recent, and decidedly so-so, MDNA: its tracks make up nearly half the set. In addition, the hits that do turn up have frequently been faffed about with in a way that speaks less of brilliant reinvention than an innate misunderstanding of what their original strengths were. She performs Like a Virgin as an agonisingly slow torch ballad. As she sings it, a man appears behind her and starts yanking at the laces of her corset as if trying to pull her offstage, giving rise to the fear that he's a representative of Westminster Council's environmental health department, taking residents' complaints about noise into his own hands. But no, the gig goes on, to a finale of Like a Prayer: the sound – albeit muffled – of Madonna playing to her own strengths.